Optical memory technology that uses optical disks that have pit-shaped patterns as high-density, large-volume memory media is gradually being applied widely to and entering general use in digital audio disks, video disks, document file disks and also data files. Thus, the functions for using a minutely narrowed light beam to successfully achieve recording onto and reproduction of information from an optical disk with high reliability, are divided into three main divisions, that is, a focusing function which forms a minute spot at the diffraction limit on the optical disk, focus control (“focus servo”) and tracking control of the optical system, and pit signal (“information signal”) detection.
With recent advances in optical system design technology and the shortening of the wavelengths of the semiconductor lasers serving as light sources, the development of optical disks containing volumes of memory at greater than conventional densities is progressing. As an approach towards higher densities, problems such as an increase in the aberration due to slanting of the light axis (what is known as “tilt”) were found when investigating an increase in the optical disk side numerical aperture (NA) of the focusing optical system that focuses light beams onto the optical disk. That is to say, the amount of aberration that occurs with respect to the tilt increases when the NA is increased. It is possible to prevent this by thinning down the thickness (substrate thickness) of the transparent substrate of the optical disk.
The substrate thickness of a Compact Disc (CD), which can be considered a first generation optical disk, is approximately 1.2 mm, and the optical head device for CDs uses a light source emitting infrared light (with a wavelength λ3 that is 780 nm to 820 nm) and an objective lens with a NA of 0.45. Furthermore, the substrate thickness of a Digital Versatile Disc (DVD), which can be considered a second generation optical disk, is approximately 0.6 mm, and the optical head device for DVDs uses a light source emitting red light (with a wavelength λ2 that is 630 nm to 680 nm), and an objective lens with a NA of 0.6. Moreover, the substrate thickness of a third generation optical disk is approximately 0.1 mm, and the optical head device for these disks uses a light source emitting blue light (with a wavelength λ1 that is 390 nm to 415 nm), and an objective lens with a NA of 0.85.
It should be noted that in this specification, “substrate thickness” refers to the thickness from a surface of the optical disk (or the optical recording medium) on which the light beam is incident to the information recording surface. As described above, the substrate thickness of the transparent substrate of the high-density optical disks is set to be thin. From the view point of economy and the space that is occupied by the device, it is preferable that an optical information apparatus can record and reproduce information from a plurality of optical disks having differing substrate thicknesses and recording densities. However for this, it is necessary to have an optical head device provided with a focusing optical system capable of focusing a light beam up to the diffraction limit onto a plurality of optical disks having differing substrate thicknesses.
Furthermore, if recording on or reproducing from an optical disk whose substrate thickness of the transparent substrate is thick, then it is necessary to focus the light beam onto the information recording surface that is further back than the disk surface, and thus the focal length must be increased.
A configuration is disclosed in JP H11-339307A (first conventional example) whose object is defined so as to provide an optical head device that records and reproduces information from a plurality of optical disks with differing substrate thicknesses. The first conventional example is described below with reference to FIG. 17 and FIG. 18.
As shown in FIG. 17, the optical head device according to the first conventional example, is provided with a mirror 31 that has a plurality of reflective surfaces with differing radii of curvature and whose reflective surfaces are constituted by a dielectric multi-layer film, and an objective lens 1805 that is designed with an aperture diameter that reproduces high density optical disks with a light beam from the shortest wavelength of the light beams 401 to 403 that are emitted from light sources of different wavelengths. Here, the wavelengths of the light beams 401 to 403 become shorter in this order, and the wavelength of the light source that is used is determined by the optical disk type. The light beam 403, which has the shortest wavelength of the light beams 401 to 403, is used if a high density optical disk 10a is reproduced, the second shortest wavelength light beam 402 is used if reproducing a medium density optical disk 10b, and the longest wavelength light beam 401 is used if reproducing a low density optical disk 10c. The light beams 401 to 403 are reflected toward the optical disk by the mirror 31, which has a plurality of reflecting surfaces with different radii of curvature, and are incident on the objective lens 1805.
As shown in FIG. 18, the mirror 31 contains a plurality of reflective surfaces with different radii of curvature for the purpose of reflecting the plurality of light beams 401 to 403 toward the respective optical disks. A first reflecting surface 311 is constituted by a dielectric multilayer film that totally reflects and converts the light beam 403 to a light bundle having an optimum spreading angle with respect to the objective lens 1805, as well as totally passing light beams emitted from the other light sources. Furthermore, a second reflecting surface 312 (a spherical surface with a radius of curvature R2) is constituted by a dielectric multilayer film that totally reflects and converts the light beam 403 to a light bundle having an optimum spreading angle with respect to the objective lens 1805, as well as totally passing light beams that are emitted from the other light sources. Furthermore, a third reflecting surface 313 (a spherical surface with a radius of curvature R3) is constituted by a dielectric multilayer film that totally reflects and converts the light beam 401 to a light bundle having an optimum spreading angle with respect to the objective lens 1805, as well as totally passing light beams emitted from the other light sources. By selecting the reflective surfaces in accordance with the wavelength of the light sources and the types of optical disks in this way, it is possible to convert the wave front of the light beams 401 to 403 to allow interchangeable reproduction of the plurality of different types of optical disks 10a to 10c. 
Furthermore, a configuration whose object is to allow interchangeable reproduction of a plurality of different types of optical disks, using a plurality of light beams having different wavelengths is also disclosed in for example JP H10-334504A (second conventional example) and in JP H11-296890A (third conventional example). That is to say, a configuration which uses a diffraction optical element (DOE) or a phase converting element combined with an objective lens is disclosed in the second conventional example. Furthermore, a configuration in which a plurality of objective lenses is mechanically interchanged is disclosed in the third conventional example.
As shown in FIG. 17, in the first conventional example, the objective lens 1805 is driven independently of the mirror 31 (see FIG. 4 to FIG. 6 of JP 11-339307A). However, in the first conventional example, because the light beam is converted, as described above, by the curved mirror 31 from parallel light to a light bundle that has an optimum spreading angle, a relative position of the objective lens changes with regard to the incident light wave front, aberrations occur, and focusing characteristics are degraded when the objective lens moves due to tracking during recording or reproduction of the optical disk. Furthermore, the reflecting surface of the mirror 31 is a curved surface, that is to say, it is made from a spherical surface. However a spherical surface is insufficient to compensate for differences in the substrate thicknesses between the optical disks 10a to 10c and differences in wavelength between the light beams, and it is not possible to diminish fifth order or higher order aberrations.
Furthermore, in the second conventional example, diffraction optical elements and phase converting elements are used such as described above. However, it is necessary to increase the focal length if recording or reproducing optical disks whose transparent substrate thickness is thick, and for this purpose, it is necessary that the optical element that converts the light beam contains a certain lens power. However, the lattice pitch becomes finer toward the outer peripheral portion when lens power is applied to the diffraction optical element and the lattice pitch is equivalent to the wavelength for example if the numerical aperture is in the order of 0.6, and as a result the diffraction effect decreases and there is a drop in light utilization. Furthermore, the structure becomes minute when a lens power is applied to a phase converting element, and the same problem occurs as with a diffraction optical element.
Furthermore, in the third conventional example as described above, a configuration in which the objective lenses are interchanged is employed, and together with an increase in the number of parts, miniaturization of the optical head device is also problematic because a plurality of objective lenses are required. Furthermore, miniaturization of the optical head device is also problematic due to the requirement of a mechanism to interchange the objective lenses.